... Driver side seatbelt warning sound won't stop even I plug in seatbelt ... What to do? ...

... If plug in passenger side, it stop ...

 

Do you carry anything on the passenger seat? any weight on it, even a little, handbag, shopping bag, book etc will set the sensor off in the seat, it thinks your driving with a passenger without their seatbelt on. If not then check under the seat to see if anything is compressing the seat sensor. It will go off if you put the seatbelt on that's normal.